Try nat (inside) 0 A.B.C.128 255.255.255.192 0 0

I'm not a PIX expert and the CL interface appears to be designed to keep said experts in employment. Anyway, that link you provided is inconsistent with regards to the nat line. In the '2 interfaces without nat' example, they give the interface address with the network submask, as suggested previously (which gives you an error). However, in the '3 interfaces without nat' example, they give the network address with its submask...